1. Field of the Invention
This invention relates to continuous stream type ink jet printers, and more particularly, to such printers which emit a plurality of ink streams, each ink stream being synchronously stimulated by electrohydrodynamic (EHD) excitation.
2. Description of the Prior Art
Ink jet devices of the continuous stream type generally employ a printhead having a droplet generator with multiple nozzles from which continuous streams of ink are emitted under pressure. The ink is stimulated prior to or during its exiting from the nozzles so that the stream breaks up into a series of uniform droplets at a fixed distance from the nozzles. As the droplets are formed, they are selectively charged by the application of a charging voltage by electrodes positioned adjacent the streams at the location where they break up into droplets. The droplets which are charged are deflected by an electric field either into a gutter for ink collection and/or reuse, or to a specific location on the recording medium, such as paper, which may be continuously transported at a relatively high speed across the paths of the droplets.
One method of stimulating the ink stream is by acoustic excitation of the ink in the ink manifold of the printhead by a piezoelectric driver. The drive produces acoustic waves which traverse the ink manifold to the nozzles, perturbing or stimulating the jets or streams and causing uniform breakup of the jets in terms of break off length and phase. Thus, the drop generator manifold or reservoir has two functions, to distribute ink to the individual nozzles and to distribute acoustic energy to the individual streams to cause a controlled uniform breakup into droplets. Another method of stimulation of the ink streams is by electrohydrodynamic (EHD) excitation. In EHD stimulated ink streams, an electrode is generally placed in the proximity of each stream a short distance downstream from the nozzle. This electrode is biased by time varying voltage in respect of the ink stream and hence it has to be electrically isolated from the ink by, for example, a dielectric spacer. The distance from the beginning of the ink stream as it exits from the nozzle to the EHD electrode is defined by the dielectric spacer. The dielectric spacer has to function as an insulator in a hostile environment, being exposed to ink vapor, ink mist, and ink contamination during start up and shutdown of the ink stream.
When a liquid stream breaks up into droplets, it rarely produces droplets of a single size. Even when the break up is stimulated by pure sinusoidal pressure, the stream normally breaks up into a series of uniform large droplets separated by a single, much smaller droplet called a "satellite". Due to the fundamental processes participating in the ink jet droplet generation, the formation of ink droplets is often accompanied by simultaneous appearance of satellites. These smaller drops or satellites are, in general, unwanted since they adversely influence the droplet charging and cause printhead contamination and print quality defects. Normally, a satellite separates first from the stream in front of the main droplet, which then separates next. Later, the satellite merges backward into the main drop, which event is called "rear merge". Because the two drops were formed at different times, they may be exposed to different charging voltages in a continuous ink jet printer. Upon recombination, the total charge of the single drop will be indeterminate, leading to placement errors on the final image. This is called "bad rear merge".
The usual way to suppress the satellite formation in the conventional acoustic ink stream stimulation is to apply the excitation to the piezoelectric transducer with such an intensity that the nonlinearities on the way from the transducer to the droplet breakoff point prevent the separation of the satellite from the main droplet by giving rise to a higher harmonics in the ink stream. It is believed that the acoustic processes in the jet nozzle are particularly important for this control.
This approach can be relatively easily implemented for a printer with a single or a few jets. It is, however, much more difficult to use this satellite control for multi-jet acoustically driven arrays. Even when the formidable associated problems are solved, the high excitation intensity inherently causes a very short droplet break off length which imposes severe constraints on the printhead architecture.
U.S. Pat. No. 3,596,275 to Sweet discloses the basic concept of an EHD exciter. The disclosed electrohydrodynamic (EHD) device requires very high voltages and expensive transformers to obtain them. The high voltages represent an electrical complexity, high cost, and safety hazard. The high voltages needed to excite or pulsate the fluid column also interferes with the subsequent droplet charging step.
U.S. Pat. No. 3,949,410 to Bassous et al discloses an EHD exciter integrated into a nozzle. In connection with FIG. 4, they describe the fundamental EHD process first articulated by Sweet in his above-mentioned patent. Bassous et al report the periodic swelling and nonswelling of a fluid column due to the electric field associated with the geometry at the nozzle orifice. They further disclose the fluid mechanics principle that the wavelength of the swelling (i.e. droplet separation) is given by the velocity of the fluid divided by the frequency of the swelling or perturbations.
U.S. Pat. No. 4,220,958 to Crowley discloses a continuous stream-type ink jet printer wherein the perturbation is accomplished by electrohydrodynamic excitation. The EHD exciter is composed of one or more pump electrodes of a length equal to about one-half the droplet spacing. The multiple pump electrode embodiments are spaced at intervals of multiples of about one-half the droplet spacing or wavelength downstream from the nozzles.
U.S. Pat. No. 4,568,946 to Weinberg discloses a charge electrode means for sensing a charge on the individual ink droplets passing thereby and providing signals which can be used to control the timing of the charging electrical pulses applied to the charging electrodes. The charge electrode means comprises a pair of electrical insulating members mounted in spaced relation to one another so as to provide a gap between opposed surfaces thereof. Conductive charge electrode layers are provided on these opposed surfaces and are electrically connected.
An article entitled "Satellite Droplet Formation in a Liquid Jet" by Pimbley and Lee, IBM Journal of Research and Development, Vol. 21, No. 1, January, 1977, pages 21-30, discloses an investigation into the formation and behavior of satellite droplets. The two most relevant parameters that control satellite droplet formation are the amplitude of the perturbation and the wavelength-to-diameter ratio of this perturbation. Satellite formation is least likely to occur when the main drop spacing is five to seven times the jet diameter.